Table 1.

Maximal Values of Rate Constants for Hydration of CO₂, Hydrolysis of 4-Nitrophenylacetate, and Proton Transfer in Dehydration Catalyzed by HCA II and Variants.^a

Enzyme	kcatexch/KeffCO2 CO2 hydration (µM−1s−1) b	kcat/Km esterase (M−1s−1) c	kB proton transfer (µs−1) d
wild type	120	2800	0.8 ± 0.1
Y7I	130	2400	2.3 ± 0.2
Y7A	140	2300	0.8 ± 0.3
Y7W	140	1700	1.8 ± 0.1
Y7F e	120	4400	7 ± 0.2
Y7D	130	1800	0.8 ± 0.1 f
Y7N	100	1200	2.5 ± 0.1 f
Y7R	120	1700	1.5 ± 0.1
Y7S	100	1600	1.0 ± 0.1

^aDerived from the kinetic curves for each substitution by a fit of to the data of Figures 2,3 and Supporting Information Figures S1–S4. All data were obtained at 25°C.

^bMeasured from the exchange of ¹⁸O between CO₂ and water using eq 5 in the hydration direction. Standard errors here were less than 10%.

^cMeasured from the fit of the rate constants for ester hydrolysis to a single ionization. Standard errors here were less than 10%.

^dMeasured from the exchange of ¹⁸O between CO₂ and water using eq 6 in the dehydration direction.

^eData are from Fisher et al. [8].

^fThese are maximal values of R_H2O/[E] since incomplete pH profiles (Figures S3, S4) did not allow an adequate determination of k_B by a fit of eq 6.